Process for the utilization of heat in ammonia oxidation-nitric acid pressure processes



Apnl l0, 1934. T. HoBLER 1,954,317

PROCESS FOR THE UTILIZATION OF HEAT IN AMMONIA OXIDATION-NITRIC ACIDPRESSURE PROCESSES Filed July 22, 1933 7 0 250% co/mwfsof/ la 8 C72Patented Apr. 10, 1934 UNITED STATES PATENT OFFICE PROCESS FOR THEUTILIZATION OF HEAT IN AMMONIA OXIDATION-QNITRIC ACID PRESSURE PROCESSESTadeusz Hobler, Paris, France, assignor to firm Hydro Nitro S. A.,Geneva, Switzerland, a corporation of Switzerland Application July 22,1933. Serial No. 681,690 In Germany January 22, 1932 i Claims. (Cl.23--162) In manufacturing nitric acid by the oxidation and then bringingthem to the desired temperaof ammonia with air and absorbing the nitricture before entering the expansion engine by oxides produced, and inother reactions of like means of the heat in the nitrogen oxides gases.nature, it is advantageous to carry on the oxida- In this case the heatused for preheating the 5 tion under pressure instead of at atmosphericresidual gases passing to the expansion engine o or low pressures. Amongother advantages, it With heat taken from the exhaust gases from theenables nitric acid of considerably greater conexpansion engine is usedover and over again centration to be obtained and occasions great Whilstthe heat that is added to the thus presavings in requirement of spaceand material. heated gases gOing t0 the eXDansiOn engine t0 at- Theseadvantages are, however, opposed by the tain the desired hightemperature is taken from 65 great consumption of energy for the Work ofthe hot nitrogen oxides gases. The temperature compression. It hasindeed been attempted to 0f the het nitlOgen OXides gases is in thisCase recover the energy of the residual gases; this has, reduced Only t0Sueh an extent that it is still suiihowever, only been partiallysuccessful hitherto, Cient fOi the pleheating 0f the lniXtule 0f am* asthe Work of compression could hardly be recovmOna and ail' t0 theOptimum temperature.

ered to as much as from 40 to 60%. In this con- It has been known toutilize the heat content nection it is to be taken into considerationthat Of the eXhaust gases 0f eXDansiOn 0f hOt gases the volume of theresidual gases is reduced by in eXDansiOn engines fOI the Dulpese Opreheatconsumption of atmospheric oxygen by the aming the eXpansiOngases- It has further been monia to form nitrogen oxides and water andby PIODOSed in the ease 0f the Plessule abselntien the absorption of thenitrogen oxides in water in 0f anir desired gases t0 heat up theunahsl'hed the absorbers. Moreover the efficiency of the resdualsasesbefore expansion by Supplying heat expansion engine and the compressoris naturally from Outside in Oi'del t0 COmpensate in this Way not 100%.to a large extent for the compression energy that The portion of theheat energy of the hot nitrois t0 be eOnSumed.

gen oxides, which has not been used for the pre- In eempalisen Withthese knOWn methods 0f heating of the mixture of ammonia and air, hasWerking, the DTOCeSs aeeOIding t0 my invention already been employed forpreheating the residual eentains the nOVel diiielentiating feature thatgases passed to expansion. In this case, however, the unahseibedresidual gases ale heated t0 a the residual gases Were preheated only tosuch considerably gleateI eXtent than has hitherto an extent that theirenergy Was not sufficient to been usualrIhis method 0i Werking Was notcover the work of compression. obvious because on the one hand it couldnot be It has now been found that the residual gases directly assumedthat the Waste heat 0f the eX- can be preheated by the heat existing inthe sysheust gases would be useful for Such a great heattem to atemperature that is so high that, on ing 0f the Compressed' gaseseenling from the their expansion, they recover the work of comabsorptionand because on the other hand diffipression completely or almostcompletely within culties were to be expected in the introduction thelimits of pressure, nve to eight atmospheres, 0f suCh highly heated@Omplessed gases into the now usable in practice. According to theinveneXnanSiOn engine patieulaly With regard t0 the tion, the residualgases that are to be Supplied constructional material and the lubricant.These to the expansion engine are preheated to such an diffleultieS areTemOVed in the present DIOeess in extent that the exhaust expansiongases of the the manner HOW t0 he described expansion engine aresufficiently hot to preheat in In Fig 1 there is dagfammatieallyrepresented the first stage the compressed gases coming from an example0f allangement 0f an apparatus Suitthe absorption whereupon thecompressed gases able fOl eailying Out the Dreeess.

are then, in the second stage, heated up to the. The combustion air iscompressed to the necesdesired degree in heat interchange With the gasessary Werking Pressure 0f, OI eXamDle, 8 atmosof combustion or productsof oxidation or reacpheres in a Pisten engine 1. This engine Can betion. The residual gases that are to be relieved of constructed, forexample, in such a manner that pressure are thus heated entirely by theheat presthe compression of the air and the expansion of 105 ent in thesystem to a temperature Whose magnithe residual gases take place in thesame cylinder. tude is determined by the particular pressure Thecompressed combustion airis carefully freed chosen, the Work ofcompression to be performed, from oil in the lter 11 and passed to theheat exand the losses occurring, by iirst preheating these changer 3 onthe way liquid ammonia injected residual gases by means of the expansiongases into the airpipe by asuitable pump 2`. The pump 11'() nitrogenoxides leaving the combustion element 4 l of the mixture of ammonia andair 2 is coupled with the piston engine 1, so that a constant proportionof ammonia and air is ensured. The mixture of ammonia and air that hasbeen compressed to 8 atmospheres and has been preheated to about 330 C.after passing through the heat exchanger 3, enters the oxidation element4. The gases that are mixed with the nitrogen oxides formed here andhave been heated to about 850 through the heat exchanger 5 and, afterthey have here given up a portion of their heat, arrive with atemperature of about 425 C. at the heat exchanger 3 which they leavewith a temperature of 204 C. after preheating the mixture of ammonia andair. The nitrogen oxides are finally cooled down in the condenser 6, andthe condensate is separated. The uncondensed nitrogen oxides areoxidized and absorbed in water in the absorbers 7 to form nitric acid.Any nitrogen oxides left in the gases are absorbed in the alkalinewasher 8, and the residual gases, free from acids or oxides, issuetherefrom at a temperature of about 25 C. They are then preheated toabout 250 C. in the heat exchanger 9, and then heated to about 780 C. inthe heat exchanger 5 and thereupon led to the expansion engine l. Theexhaust gases issuing at about 373 C. from the expansion engine arecarried away after passing through the heat exchanger 9. The Valves 12and 13 serve for regulating the temperature of the residual gases andrespectively. 'I'he pipe 14 and the valve 15 serve for passing in thesuperheated steam which serves for setting the piston engine intooperation. The check valve 16 prevents the striking back of the steam.

A Working pressure of 8 atmospheres is the basis for the approximatetemperatures given by way of example in connection with this workingdiagram. The preheating temperature for the mixture of ammonia and airthe temperature of the and also the temperature of the relieved residualgases leaving the expansion engine may be kept at about the orders ofmagnitude given even with other working pressures; the temperature ofthe gases that are to be passed into the expansion engine, whichtemperature is to be adjusted to the particular working pressure, can becorrespondingly regulated. The other temperatures indicated shiftthemselves in accordance with the l pressure chosen and the desiredtemperature before the expansion engine is adjusted to the particularpressure; obviously, however, another temperature may be made the basisfor the relieved gases. When used for higher pressures than 8atmospheres the possible recovery of energy will not be as complete but,in any case, greater than if my invention were not used. The power ofthe heat exchangers is adjusted to the particular xed workingconditions. The number of heat exchangers as well as the passage of thegases through them may be differently combined.

According to the above example, the fall in temperature of the residualgases T1 being the temperature of the gases entering the engine 1 and T2being the temperature of the exhaust gases from the engine.

If under otherwise similar conditions the gases l leaving the alkalinewasher were led directly into heat exchanger 5 without the use ofproposed heat exchanger 9, then the temperature T1 would be only 780C.-250 C.+25 C.=555 C. (250 C. being the preheating that would have C.by the heat of reaction, pass taken place in exchanger 9 had it beenused). Then when the gas is expanded from 8 atmospheres and 555 C. to 1atmosphere, the exhaust temperature would be 232 C. and the temperaturedrop in the expansion machine would be only 555 C.-232 C.=323" C.instead of 407 C. as in the preceding example. Under these circumstancesthe available heat would not be suicient in the expansion machine tocover the work of compression. (Theoretical adiabatic energy availablebeing AL=Cv(T1-Tz).

Owing to the high initial temperatures of the residual gases which formthe basis of the present process, many oliicultes occur particularlywith regard to finding a constructional material which is suilicientlyheat-resisting and at the same time has all the properties that arenecessary in engine construction. Similar dimculties are encountered inthe selection of the lubricant.

In the case of smaller working pressures and, consequently, smallerinitial temperatures, the difficulties mentioned can be removed forexample by the employment of a reaction gas turbine the fall intemperature of the gases that is dependent upon the fall in pressurebeing already effected on issuing from the nozzles. A sufficientlyheat-resisting material for the stationary nozzles can be found withoutdimculty. All the remaining portions of the turbine are, however,exposed merely to the temperature of the expanded gases, so that thedifficulties indicated can be overcome.

These diiiiculties are, however, also removed by the application of apiston engine in such a manner that the expansion of the residual gasesand the compression of the air mixture for the combustion are carriedout in the same cylinder and that each expansion period of the residualgases alternates with a compression period of the combustion air, withthe result that a mean temperature that is suiiiciently low both for theconstructional material as well as for the lubrication is ensured in thecylinder.

Such a method of Working can readily be carried out in, for example, afour-stroke cycle. The theoretical diagram of Fig. 2 represents such anoperation.

In the rst stroke, from the point A to the point B, the combustion airwith a temperature of about 25 C. is drawn in; in the second stroke,from the point B to the point C, a portion of the volume of air that hasbeen drawn in is ejected and the compression of the combustion air iscarried out in the further portion of the stroke; iinally, between thepoints D and E the compressed air is pushed into the pressure piping. Inthe iirst section of the third stroke at the point E the flowing in ofthe residual gases begin. The filling is eiTected up to the point F andthe expansion is effected from F to G; the flowing forward takes placefrom the point C to the point B and the discharge takes place from B toH.

As may be seen, the iirst and second strokes correspond except for thepath B-C to a compression diagram and the third and fourth strokescorrespond to a normal diagram of an expansion engine.

The diagram described is purely theoretical; in practice the workingiield of the diagram is smaller. The shaded area D, F, G, B, C rpresentsthe positive work and the area E, E', D, D and the surface H, A, Brepresent the negative work; the diierence illustrates the excess ofwork for covering the losses.

It is further pointed out that in the residual gases issuing from thealkaline washing only very small traces of NO2 are contained and thateven these are dissociated at the high temperature employed so that thegases, which contain merely traces of NO, do not cause any lcorrosion.

The method of working described can also be employed in the case inwhich the combustion of the mixture of ammonia and air is carried outWithout pressure and only the absorption is carried out under pressure.In this case, the compression of the nitro gases can be carried out by aturbo-compressor which has already been proposed for this purpose andfor which the motive power is supplied by a reaction gas turbine that isdriven with the residual gases in the manner described.

4What I claim as my invention and desire to secure by Letters Patent ofthe United States of America isz- 1. In the production of nitric acidunder pressure by the oxidation of ammonia by air under pressure,absorption of the resulting oxides, and utilizing the residual gases inan expansion engine to compress air for said combustion, the processwhich comprises heating said residual gases from said absorption rst byabstraction of heat from the exhaust gases from said expansion engine,then by abstraction of heat from the products of said oxidation.

2. In the production of nitric acid under pressure by the oxidation ofammonia by air under pressure, absorption of the resulting oxides, andutilizing the residual gases in an expansion engine to compress air forsaid combustion, the process which comprises heating said residual gasesfrom said absorption first by abstraction of heat from the exhaust gasesfrom said expansion engine, then by abstraction of heat from theproducts of said oxidation, then compressing air for said oxidation byexpanding said heated residual gases in the expansion engine.

3. In the production of nitric acid under pressure by the oxidation ofammonia by air under pressure, absorption of the resulting oxides, andutilizing the residual gases in an expansion engine to compress air forsaid combustion, the process which comprises heating said residual gasesfrom said absorption rst by abstraction of heat from the exhaust gasesfrom said expansion engine, then by abstraction of heat from theproducts of said oxidation, then compressing air for said oxidation byexpanding said heated residual gases in the expansion engine and heatingsaid compressed air prior to said oxidation by abstraction of residualheat from the products of combustion after the abstraction oi heattherefrom by said residual gases.

4. The process of claim 3, in which the ammonia is mixed with said airprior to heating.

5. In the production of nitric acid under pressure by the oxidation ofammonia by air under pressure, absorption of the resulting oxides, andutilizing the residual gases to compress air for said combustion, theprocess which comprises heating the residual gases from said absorption,compressing a supply of air by said heated residual gases in alternatecycles in a common cylinder whereby the mean temperature of the combinedcycles is loW relatively to that of said heated residual gases.

TADEUSZ HOBLER.

